Light-emitting devices such as light-emitting diodes (LEDs) have features such as low power consumption, small size, and light weight, and light-emitting devices encapsulated with resins are used in various types of lamps and the like. In recent years, blue LEDs and white LEDs have been developed, and the luminance of LEDs has also been increased. Hence, the use of LEDs is expanding rapidly in applications such as a backlight light source for liquid crystal display panels, a light source for illumination, and signal lamps. Furthermore, the application of LEDs to headlamps for use in automobiles is being developed.
Conventionally, epoxy resins of bisphenol A glycidyl ether type having a refractive index of 1.53 to 1.57 have been used as an encapsulating resin for LEDs. However, with the increase in luminance of LEDs, the temperature of the LEDs during operation increases, and therefore the encapsulating material for the LEDs is exposed to high temperature and high intensity light. In such a circumstance, the heat resistance and light resistance (in particular, resistance to UV and blue light) of conventional encapsulating resins composed of epoxy resins are insufficient. Therefore, the encapsulating resins are discolored, and this causes a problem that the luminance of the LEDs deteriorates with time. With regard to this problem, highly transparent epoxy resins have been developed. However, the heat resistance and light resistance of such resins are still not satisfactory.
In view of the above, gel-type silicone resins have been used in high-intensity LEDs since they are excellent in heat resistance and light resistance as compared with epoxy resins. However, the gel-type silicone resins have the following problems.
First, the surface of these silicone resins is sticky, so that a problem arises in that dust and dirt easily adhere thereto. Therefore, at present, the silicone resins are limited to be used as a resin for filling a gap formed after a dome portion having a lens function is joined with the base of an LED chip and to be used as an encapsulating resin when an LED is to be surface mounted.
Second, since the refractive index of the silicone resins is in the range of 1.41 to 1.51 being smaller than that of epoxy resins, the light extraction efficiency from LEDs deteriorate when the silicone resins are used for encapsulation. Specifically, in high intensity LEDs, a sapphire substrate is often used as a chip substrate thereof, and a certain method is mainly employed in which light is extracted from the sapphire substrate side. The refractive index of sapphire is 1.76. Thus, in order to efficiently extract light from the sapphire substrate into an encapsulating resin, it is preferable that the refractive index of the encapsulating resin be close to the refractive index of sapphire. However, among the silicone resins, generally used dimethyl silicone resin has a refractive index of 1.41. Furthermore, diphenyl dimethyl-based and phenyl methyl-based silicone resins, into which a phenyl group has been introduced to increase the refractive index, have a refractive index of about 1.51. Thus, the refractive index of such silicone resins is less than that of epoxy resins being in the range of 1.53 to 1.57. Therefore, when a silicone resin is used as the encapsulating resin for high-intensity LEDs, it is inevitable that the light extraction efficiency is lower than that when an epoxy resin is used.
Third, since the silicone resins used in electronic materials are of an addition reaction type and are a two-part resin, the two parts are required to be mixed immediately before use. Generally, the two parts are mixed using a static mixer. However, this mixer can mix only relatively low viscosity materials, and therefore it is difficult to obtain a resin composition having a sufficiently high viscosity after the mixing of the two parts. Hence, such resins cannot be molded into a predetermined lens shape, and a lens function cannot be imparted to the encapsulating resins.
A technique has been proposed for the refractive index problem among the problems in the silicone resins (Patent Document 1). Specifically, in this technique, the refractive index of a resin composition is increased by adding fine particles of titanium oxide, zirconium oxide, zinc oxide, or the like having a high refractive index to the resin. However, in order to increase the refractive index of the silicone resins to higher than that of epoxy resins by means of this technique, at least 10 to 40% by volume of the fine particles must be added to the resin. Therefore, the addition of the fine particles rather deteriorates the transparency. Moreover, it is difficult to obtain fluidity necessary for use as an encapsulating resin. Furthermore, there is an attempt to improve the transparency by using fine particles called single-nano size. However, the cohesive force of the ultrafine particles of single-nano size is very strong, and therefore it is very difficult to uniformly disperse the ultrafine particles in a resin without formation of secondary aggregated particles. Therefore, a technology for encapsulating LEDs with a resin in which such fine particles are used has not been practically realized.
Meanwhile, it has been proposed that a fluorene group-containing monoacrylate is used as a high-refractive index resin used in the manufacturing of an optical antireflective film (Patent Document 2). This compound may be considered to be used as an encapsulating resin for LEDs.
However, since fluorene group-containing monoacrylates have very high viscosity, its handleability as an encapsulating agent is insufficient. When a low-viscosity diluent monomer such as 2-hydroxyethyl acrylate is added to a fluorene group-containing monoacrylate, the viscosity of the composition can be reduced. However, a problem arises in that the use of the diluent monomer limits the refractive index to at most 1.58 to 1.61. Furthermore, this composition is very hard after curing. Hence, when the composition is used as an encapsulating resin for LEDs, thermal stress may cause problems such as peeling between the resin and the chip, breakage of the chip, and a break in wiring.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-15063.    [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-293762.